You’re Cersei’s daughter. After Myrcella’s death she is desperate to protect you from the capital. So she sent you to Willas. You are on your way to The Reach, your mother set a marriage proposal to Willas, hoping he -since he is “crippled” and you are a royal, sees it as an honor- would accept you as wife, and you get to live the rest of you life in the peaceful lands of the Reach, and not under the cruel man of the capital.